Citrus Leafminer

AGFACTS AGFACTS Citrus leafminer AGFACTSwww.dpi.nsw.gov.au Agfact H2.AE.4, fourth edition 2004 Andrew Beattie, Professor of Agricultural Entomology, University of Western Sydney Sandra Hardy, Technical Specialist Citrus, Gosford

Citrus leafminer, Phyllocnistis citrella, is a small moth Leafminer is native to eastern and southern Asia and is pest of citrus. Damage is caused by the larvae as they now widely distributed where citrus is grown in Asia, mine immature foliage. Twisted and curled leaves are Papua New Guinea, Australia, northern and central generally the first symptoms noticed. Severe Africa, and Florida in the United States of America. It infestations—an average of two or more mines per has been linked to the severity of citrus canker leaf—can retard the growth and yield of nursery and (Xanthomonas axonopodis pv. citri), a serious disease of newly planted trees, but their effect on mature trees is citrus. less serious. Such infestations usually only occur in late The moth was first recorded in Australia in and summer and autumn, and are often related to low around Darwin in the Northern Territory in 1912. It natural enemy activity. They rarely occur in spring was probably present several or more years earlier and because the production of new growth is prolific was apparently eliminated in 1922 after a five-year and synchronised, and quickly becomes immune to campaign to eradicate citrus canker. During the attack. campaign all citrus trees north of the nineteenth parallel in the territory were destroyed. Since then rigorous quarantine measures have been in force to prevent establishment of the disease in Australia. In 1940 citrus leafminer was again found in Darwin and in 1965 it was recorded at Cairns in Queensland. Between 1965 and 1985 it spread slowly southwards along the east coast of the continent to the NSW south coast. In 1988 it was still only found in the Northern Territory and in the coastal districts of NSW and Queensland, and was affecting less than 20% of the Australian citrus industry. In 1989 it was recorded in inland NSW for the first time in orchards and home gardens from Dubbo north. Between 1989 and 1992 it spread westward into the Murrumbidgee Irrigation Areas, to Hillston, and to the NSW and Victorian River Murray districts east of Severe damage on heavily infested pummelo leaves. and including Swan Hill. By January 1993 it had

ORDER NO. H2.AE.4 AGDEX 220/622 reached the NSW/Victorian Sunraysia and South dark excreta. These larvae never leave their mines to Australian Riverland districts. By April 1995 it had form other mines or move between lower and upper spread across the continent to south-western Western sides of leaves. Development of three stages takes Australia. about five or six days in summer. Mature third instar larvae are about 3 mm long. DESCRIPTION AND BIOLOGY Under favourable summer and autumn conditions a The fourth instar (prepupa) is yellowish-brown and generation (adult to adult) is completed in 14 to 17 resembles the third instar larva but it does not feed. It days. In late autumn, winter and spring it can take two lasts for about one day in summer and uses silk or three times longer. produced from its mouthparts to form a pupal chamber. This chamber is usually located on the leaf Adults. The adults are small, delicate moths with margin, the edge of which is rolled over so that three narrow paired forewings and hindwings fringed with sides of the chamber are formed from fresh leaf long hairs. The upper surface of each forewing has a tissue and one by a thin layer of mined tissue. The black dot at the tip, and an irregular dark line chamber is formed as the delicate layer of silk separating an inner region (covered with silvery scales) produced by the prepupa dries and contracts. The from an outer region (with silver, light yellow and exposed portion of the chamber often has a distinct brown scales). The hindwings are narrower than the orange colour but is otherwise similar to the colour forewings and are covered with silvery scales. of the mined leaf surface. At rest, with their wings folded lengthwise, the moths Pupae. The yellowish-brown pupae are about are about 2 mm long. In flight, their wingspan is 2.5 mm long. After about six days a small opening is about 4.5 mm. made at the anterior end of the pupal chamber and Because flight generally occurs at night, the adults are the pupae force themselves partially through the rarely seen in daylight except when they are disturbed, openings. The adults then emerge and the cast pupal generally by human activities. When this occurs, flight skins usually remain protruding from the chambers. is short and rapid. Females are slightly more common than males. Both sexes emerge from their pupal stage during the early morning hours. Mating generally occurs at dusk and in the early evening about 9 to 12 hours after emergence. Adult female moths start laying eggs about 24 hours after mating. A female can lay more than 50 eggs during her life and as many as 20 per night. Most adults live for less than a week but they can live for up to 20 days. Eggs. The flat, slightly oval eggs are about 0.3 mm long. They are translucent but appear light green because of the leaf surface. Eggs are laid singly, Two eggs adjacent to the midrib on the lower surface of generally during the evening, on the underside of a leaf. leaves near the midrib; egg deposition on the upper leaf surface is usually associated with very humid conditions and moderate or severe infestations. Hatching can occur within 1 day in summer and the young larva immediately burrows under the surface of the leaf. Young leaves 10 to 20 mm in length are preferred sites for egg laying. Eggs are rarely laid on leaves longer than 40 mm. Larvae. There are four larval instars (stages). The first three feed only on sap from epidermal cells ruptured by their blade-like, finely toothed mouthparts. Each pale green larva forms, due to air and condensed Mature third instar larva feeding on sap from ruptured water vapour, a characteristic silvery, serpentine mine epidermal cells; note silvery appearance of mine lined with a raised parchment-like skin lined centrally with centrally with excreta (magnifications: 5.75× and life size).

2 Pupa removed from pupal chamber and leaf edge rolled inwards to form a pupal chamber.

CONTROL Cultural control Because infestations are restricted to flush growth, Semielacher petiolatus (adult female). particularly in late summer and autumn, their severity can be reduced by: • fertilising in winter to promote flush growth in spring when the pest is either absent or relatively scarce, and • limiting flush growth in late summer and autumn by not fertilising and irrigating during summer and autumn in excess of the amount needed for normal growth. Pruning of late summer and autumn flush growth can also be used to limit and remove unsightly infestations on home garden trees. Natural enemies These include small parasitic wasps and predators such as lacewings. The predators are generally associated with heavy infestations. Three of the most effective wasps are Ageniaspis citricola and Cirrospilus quadristriatus (both introduced from South-East Asia in 1990–92) and Semiolacher Ageniaspis citricola (female and three pupae). petiolatus, a native species. Parasitism by other wasps native to Australia (Cirrospilus near ingenuus, Sympiesis sp. and Zaommomentedon brevipetiolatus) has also been observed. In the Riverina, the Sunraysia and the Riverland, S. petiolatus is the major parasite. Parasitism levels are generally below 20% of all larvae in January– February, but build to 50% or more in March–May. In Queensland, the levels of parasitism of larvae by A. citricola reach 90% by February-March, but S. petiolatus and C. quadristiriatus are also important. Chemical control Effective control using chemicals is difficult because larvae are protected by their mines and pupae are protected by their pupal chambers. Insecticides, such Cirrospilus quadristriatus (adult female and pupa).

3 as organophosphates, carbamates, and pyrethroids, In addition to their effectiveness, the major advantages also disrupt the activity of natural enemies. Such of using HMOs and AMOs are: disruption can lead to outbreaks of other pests (e.g. • They can be handled with minimum protective scales and mites). clothing such as overalls, goggles and a Sprays are usually only required for control on young facemask. or vigorous trees in summer and autumn. Spraying • They have low toxicity to vertebrate animals— immature flush on mature trees is generally only they are almost as pure as the products used for baby and hair oils, skin lotions and creams. warranted for aesthetic reasons or to protect prolific growth that may occur if trees are heavily pruned in • They have little detrimental effect on beneficial insects and mites. summer. • They do not stimulate outbreaks of pests. Spraying should commence as soon as the summer • Pests have not developed resistance. flush commences in mid to late January, before the • Spray deposits are broken down within weeks first flushes produced by trees attain a length of by microbes, oxidisation and ultraviolet light to 10 mm. This strategy will prevent rapid growth of form simple molecules that do not pose a threat leafminer populations and reduce the risk of heavy to the environment. infestations during peak periods of flush growth later Guidelines for using HMOs and AMOs in the season. Spraying should cease when most of the Use concentrations ranging from 250 mL to 500 mL leaves produced within a flush cycle start to harden or of oil per 100 L water (25–50 mL per 10 L water; are longer than 40 mm. 0.25–0.5%). Use products formulated for dilution in Sprays should be applied thoroughly to the upper and water rather than products formulated for use lower surfaces of susceptible leaves. They should without mixing in water. generally be applied at 7-day intervals during warm to Sprays should be applied at volumes sufficient to wet hot weather, and every 10 to 14 days during cooler the upper and lower surfaces of susceptible leaves to periods. The number of sprays required depends on the point of run-off. They should be applied at 5 to the citrus cultivar being treated and the duration of 7-day intervals during warm to hot weather, and flushing. Lemon trees will generally require more every 10 to 14 days during cooler periods. The sprays than orange, mandarin or grapefruit trees. number of sprays required depends on the citrus Horticultural mineral oils (HMOs) and agricultural cultivar being treated and the duration of flushing. mineral oils (AMOs) are generally as effective as insect Lemon trees will generally require more sprays than growth regulators and broad-spectrum orange, mandarin or grapefruit trees. organophosphates and carbamates. They are the only In orchards, rotary atomiser and oscillating boom products recommended for general use in nurseries, sprayers are more effective than conventional airblast home gardens, and orchards. Following an sprayers. Spray volumes required for effective control international conference on spray oils in Sydney in will generally be 70% less than those required for 1999 these names were recommended and adopted control of armoured scales and soft scales. to replace ‘petroleum spray oils’ and ‘white oils’. HMOs and AMOs are high quality mineral oils Precautions and compatibilities refined from virgin distillates. Sprays applied to both HMOs and AMOs may damage plants (phytotoxicity) susceptible and mature leaves will control a range of but this rarely occurs with modern formulations and others pests (e.g. armoured scales, mites and some can be avoided by correct use. thrips) simultaneously and improve control of citrus • Aim to have the oil spray dry on the plant leafminer. within 1-2 hours of application. The oils are effective because adult female moths • To prepare an oil spray, fill the spray tank with two-thirds of the water, add the oil whilst avoid sprayed surfaces and this leads to reduced egg agitating the tank, then top up with the laying. Broad-spectrum insecticides and growth remaining water. regulators are generally used to kill adults and • During application oil–water mixtures held in immature stages, or to disrupt the development of spray tanks or containers must be continually some immature stages. and effectively agitated or stirred to prevent the oil separating from the water. Do not let the spray mix stand longer than 10 minutes. If you do, then rigorously agitate or stir before recommencing spraying.

4 • Do not apply too much oil annually. At rates Acknowledgments recommended under Guidelines for Using Recommendations for the use of HMOs and AMOs HMO’s and AMO’s do not use more than 3L of in this Agfact are based on research funded by the oil annually in Queensland and South Australia or more than 4.5L of oil in New South Wales and Horticultural Research and Development Corporation Victoria. Use less oil if higher spray volumes are (now Horticulture Australia Limited), the NSW used. Horticultural Stock and Nurseries Act Ministerial • Do not apply oils in temperatures higher than Advisory Committee, the Australian Centre for 35°C. Take care when applying sprays when the International Agricultural Research, NSW Agriculture, ambient shade temperature exceeds 32°C. the University of Western Sydney, Ampol Limited and • Do not apply sprays containing more than 250 Caltex Oil (Australia) Pty Ltd. mL of oil per 100 L water for at least one month after spraying with sulfur. • HMOs and AMOs are compatible with copper sprays up to a concentration of 0.5%. • Do not spray when the soil is dry; trees must not be suffering from moisture stress. • Always store oils in sealed drums, cans or brown bottles in a cool dark or dimly lit place. Oils stored in clear glass or plastic bottles will oxidise. Acids formed as a result of this process can be phytotoxic and ‘burn’ foliage and fruit, or, because they are hygroscopic, lead to them accumulating water on sprayed surfaces. • Excessive use of mineral oil sprays can reduce yields by clogging up the water and food transport systems of the tree. • Do not use oils that do not meet standards required for HMOs and AMOs. Such products oxidise readily. For more information on oils, see Agfact H2.AE.5 Citrus petroleum-based spray oils.

ISSN 0725-7759

DISCLAIMER The information contained in this publication is based on knowledge and understanding at the time of review December 2004. However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of New South Wales Department of Primary Industries or the user’s independent adviser.

This publication is based on the second edition of the Agfact by GAC Beattie (when Special Entomologist at NSW Agriculture) and D Smith (Principal Entomologist, Queensland Department of Primary Industries, Maroochy Horticultural Research Station Nambour).

Old damage on mature leaf. jn 5316